The climate varies due to human activity, natural climate cycles, and natural events external to the climate system. Understanding the different roles played by these drivers of variability is fundamental to predicting near-term climate change and changing extremes, and to attributing observed change to anthropogenic or natural factors. Natural drivers such as large explosive volcanic eruptions or multidecadal cycles in ocean circulation occur infrequently and are therefore poorly represented within the observational record. Here we turn to the first high-latitude annually-resolved and absolutely dated marine record spanning the last millennium, and the Paleoclimate Modelling Intercomparison Project (PMIP) Phase 3 Last Millennium climate model ensemble spanning the same time period, to examine the influence of natural climate drivers on Arctic sea ice. We show that bivalve oxygen isotope data are recording multidecadal Arctic sea ice variability and through the climate model ensemble demonstrate that external natural drivers explain up to third of this variability. Natural external forcing causes changes in sea-ice mediated export of freshwater into areas of active deep convection, affecting the strength of the Atlantic Meridional Overturning Circulation (AMOC) and thereby northward heat transport to the Arctic. This in turn leads to sustained anomalies in sea ice extent. The models capture these positive feedbacks, giving us improved confidence in their ability to simulate future sea ice in in a rapidly evolving Arctic.

Hamby, H., Li, B., Shinopoulos, K.E., Keller, H.R., Elliott, S.J., Dukovic, G., 2020. Light- driven carbon-carbon bond formation via CO reduction catalyzed by complexes of CdS nanorods and a 2-oxoacid oxidoreductase. 2 Proceedings of the National Academy of Sciences 117, 135-140.

https://doi.org/10.1073/pnas.1903948116 Significance: Nature uses enzymes to catalyze a vast array of complex chemical reactions. Enzymes that catalyze reduction reactions require partners that provide the electrons ultimately used in catalysis. In nature, the associated electron transfer sequences can be complicated, energy-inefficient, and rate determining. Here, we employ artificial electron donors, photoexcited semiconductor nanocrystals, to provide the electrons for catalysis. We couple these nanocrystals with an enzyme that catalyzes the fo rmation of carbon-carbon bonds via CO reduction. With this architecture, we demonstrate light-driven formation of 2-oxoglutarate, storing 2 some of the photon energy in the chemical product. We examine the role of electron transfer from the nanocrystal to the enzyme in the light-driven chemical conversion and learn that the enzyme modulates this process during catalysis. Abstract: Redox enzymes are capable of catalyzing a vast array of useful reactions, but they require redox partners that donate or accept electrons. Semiconductor nanocrystals provide a mechanism to convert absorbed photon energy into redox equivalents for enzyme catalysis. Here, we describe a system for photochemical carbon-carbon bond formation to make 2 -oxoglutarate by coupling CO with a succinyl group. Photoexcited 2 electrons from cadmium sulfide nanorods (CdS NRs) transfer to 2-oxoglutarate:ferredoxin oxidoreductase from Magnetococcus marinus MC- 1 (MmOGOR), which catalyzes a carbon-carbon bond formation reaction. We thereby decouple MmOGOR from its native role in the reductive tricarboxylic acid cycle and drive it directly with light. We examine the dependence of 2-oxoglutarate formation on a variety of factors and, using ultrafast transient absorption spectroscopy, elucidate the critical role of electron transfer (ET) from CdS NRs to MmOGOR. We find that the efficiency of this ET depends strongly on whether the succinyl CoA (SCoA) cosubstrate is bound at the MmOGOR active site. We hypothesize that the conformational changes due to SCoA binding impact the CdS NR-MmOGOR interaction in a manner that decreases ET efficiency compared to the enzyme with no cos ubstrate bound. Our work reveals structural considerations for the nano-bio interfaces involved in light-driven enzyme catalysis and points to the competing factors of enzyme catalysis and ET efficiency that may arise when complex enzyme reactions are driven by artificial light absorbers.

Hampton, H.G., Watson, B.N.J., Fineran, P.C., 2020. The arms race between bacteria and their phage foes. Nature 577, 327-336.

https://doi.org/10.1038/s41586-019-1894-8 Bacteria are under immense evolutionary pressure from their viral invaders - bacteriophages. Bacteria have evolved numerous immune mechanisms, both innate and adaptive, to cope with this pressure. The discovery and